A synthetic paper obtained by stretching a polypropylene film containing from 8 to 65% by weight of an inorganic fine powder has recently been developed and is in practical use as disclosed in JP-B-46-40794 (corresponding to U.S. Pat. Nos. 4,318,950 and 4,075,050), JP-A-56-141339 and JP-A-57-181829 (the term "JP-B" as used herein means an "examined published Japanese patent application", and the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Synthetic paper of this type has a microstructure in which fine voids are formed around inorganic fine powder and a vast number of streaking cracks are formed on the surface. Synthetic paper having a structure such as this is not only lightweight but has excellent printing ink receptivity, pencil writability, water resistance, etc.
Various improvements have hitherto been made in the above-described synthetic paper. For example, an aqueous solution of an anti-electrostatic acrylic copolymer, polyethyleneimine, etc. is coated thereon at a dry spread of from 0.005 to 1 g/m.sup.2, followed by drying to produce improved offset printability, as disclosed, e.g., in JP-A-50-10624, JP-A-57-149363, and JP-A-61-3748.
A polypropylene film without any inorganic fine powder on the surface of the synthetic paper to provide high-gloss and printable synthetic paper has been developed (refer to JP-A-61-3748) or a uniaxially stretched styrene-polypropylene copolymer film containing 0 to 3% by weight of an inorganic fine powder on the surface of the synthetic paper, with the styrene-polypropylene copolymer film having a thickness one-tenth to equal to the average particle size of said inorganic fine powder to thereby prevent difficulties arising from paper dust is known as disclosed in JP-A-62-249741.
These synthetic papers possess characteristics which are not possessed by conventional paper such as water resistance, chemical resistance and mechanical strength yet they have conventional paper properties such as whiteness, opaqueness and smoothness and are used in posters, calendars, books, maps, labels, and various advertising media, neatly printed in full color by offset printing, gravure printing, screen printing, and the like.
However, in spite of the above-mentioned superiority, polypropylene-based synthetic paper has a disadvantage in that hydrocarbon compounds, particularly hydrocarbon solvents contained in quantity in inks used in offset printing, gravure printing and screen printing swells the polypropylene resin and curling occurs. Eventually this results in an unevenness due to swelling that makes the printed matter invaluable.
The swelling of synthetic paper with solvents is explained below in detail taking offset inks for instance into consideration. Because multicolor printing can be conducted with ease, offset printing is used for printing on art paper, fine paper, coated paper, polyethylene terephthalate films, polyvinyl chloride films, etc. General purpose offset inks (hereinafter referred to as GP offset inks) usually employed in offset printing comprise the following basic components in the compounding ratio shown in Table 1 below. ##STR1##
The composition of a special offset ink developed for non-absorbing materials (hereinafter described) is also shown in Table 1 below.
TABLE 1 ______________________________________ Offset Ink for Non-absorbing GP Offset Ink Materials Component (parts by weight) (parts by weight) ______________________________________ Pigment 15 to 50 (20) (25) Resin 25 to 30 (30) (30) Drying oil 10 to 15 (10) (34) High-boiling 20 to 45 (35) (5) petroleum solvent Dryer 0.5 to 2 (1) (2) Others 2 to 5 (4) (4) Total weight 100 100 ______________________________________
The numbers in parentheses indicate the composition of an example of commercially available inks.
In order to reduce drying time, a drying oil compounded with a resin and a mineral oil (high-boiling petroleum solvent) has been used as a vehicle to provide a quick-drying GP offset ink.
Where synthetic paper comprising a stretched polyolefin film containing an inorganic fine powder, e.g., the film disclosed in JP-B-46-40794, JP-A-62-227933, JP-A-61-3748, and JP-A-60-79951, is printed with the above-described GP offset ink, the polyolefin swells due to the solvent present in the ink, particularly high-boiling petroleum solvents (e.g., mineral oils), and the printed synthetic paper suffers from a so-called solvent attack, i.e., local unevenness of the surface or curling as a whole. Thus, the practical application of such GP offset inks to offset printing on polyolefin films is difficult.
In order to avoid such a solvent attack, a special offset ink with a reduced amount of high-boiling petroleum solvent as shown in Table 1 above is now employed for particular use on non-absorbing materials such as polyolefin films.
However, where printing is carried out by using the above-described special offset ink for polyolefin films, printers encounter difficulties in controlling the balance between the amount of damping water and that of the ink as compared with use of GP offset inks and are also confronted with a problem of variation in ink viscosity on printing machines, which deteriorates workability. For these reasons, printers are unwilling to use these kind of printing inks and, as a result, printers and ink manufactures who practice printing of synthetic papers are naturally limited. Therefore, development of polyolefin synthetic paper on which GP offset inks can be used without problems has been desired.
Since in practice general printers carry out, as a regular routine, offset printing on pulp paper, such as fine paper and coated paper, with a GP offset ink, they must change the GP offset ink for the special offset ink for non-absorbing materials each time polyolefin synthetic paper is to be printed. Considering much time and labor are required for the ink change, general printers avoid printing of polyolefin synthetic paper, and this has interfered with the spread of offset printing on polyolefin synthetic paper.